Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable material, most commonly of parts made of thermoplastic polymers. During a repetitive injection molding process, a plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now-molten polymer or resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to a flow channel through one or more gates that direct the flow of the molten resin into the cavity. Thus, a typical injection molding procedure comprises four basic operations: (1) heating the plastic in the injection molding machine to allow it to flow under pressure; (2) injecting the melted plastic into a mold cavity or cavities defined between two mold halves that have been closed; (3) allowing the plastic to cool and harden in the cavity or cavities while under pressure; and (4) opening the mold halves to cause the part to be ejected from the mold.
The molten plastic resin is injected into the mold cavity and the plastic resin is forcibly pushed through the cavity by the injection molding machine until the plastic resin reaches the location in the cavity furthest from the gate. The resulting length and wall thickness of the part is a result of the shape of the mold cavity.
Multi-cavity injection molds require a network of feeder channels to distribute molten plastic from the machine nozzle to each individual mold cavity. The feeder channels or runners can be permitted to cool, or can be actively cooled, such that for each molding cycle the runners are filled with molten polymer that solidifies in the runners, and is then removed from the mold as a solid mass of plastic in the shape of the runner or feeder channel network. This type of system is referred to in the art as a “cold runner” system. It is also possible to heat the feeder channel or runner network, such that for each molding cycle the polymer remains molten. The molten polymer remains in the feeder channels or runners after each molding cycle—this molten material is then injected in to the mold cavity upon initiation of the subsequent molding cycle. This type of system is referred to as a “hot runner” system. A “runner system” as used herein, if not preceded by the adjective “hot” or “cold”, refers to either a hot runner system or a cold runner system, as well as to a hybrid “hot-to-cold” runner system.
In the case of a cold runner system, the hydraulic diameter of the runner or feeder channel in closest proximity to a mold cavity typically ranges from about 1.5 to about 3 times the nominal wall thickness of the molded article. See, e.g., Beaumont, Runner and Gating Design Handbook, second edition, page 152, Hanser Gardner Publications, 2007. Hydraulic diameter, or DH, is a term commonly used in the art to refer not only to the inner diameter of round tubes, but also to an effective inner diameter of non-circular tubes or channels, which may be calculated by the formula=4A/P, where A is the cross-sectional area of the tube or channel and P is the wetted inner perimeter of the cross-section of the tube or channel. This hydraulic diameter is intentionally greater than the article nominal wall thickness (a term defined hereinafter), so that the runner will remain molten longer than the molded part, ensuring that molten plastic can continue to be fed through the feeder network until the mold cavity is completely filled and packed. If polymeric material within the feeder channel were to freeze prior to the mold cavity being completely packed, the molded article would shrink away from the mold cavity excessively, and the molded article would have poor surface finish quality and undesirably high dimensional variation.
In another convention for sizing cold runners, the runners are designed to have a hydraulic diameter of 1.5 mm greater than the nominal wall thickness of an article to be molded. See, e.g., How to Make Injection Molds, Third Edition, page 153, Carl Hanser, 1993 (Germany).
In designing runner systems, conventional design parameters call for the runner to begin near the injection unit machine nozzle at a larger cross-sectional area, then progressively step down in cross-sectional area, as the runner is divided to achieve the desired number of runners to reach each individual mold cavity. Importantly, conventional wisdom indicates that the flow runner hydraulic diameter must be increased from a minimum design hydraulic diameter (as determined above) that feeds the mold cavity, to a progressively increasing hydraulic diameter at each branch in the runner along the flow path back to the machine nozzle. This is particularly the case for cold runner systems, as in hot runner systems, since there is not the same need to promote freeze-off and minimize scrap of polymeric material within the runners by minimizing runner diameter as there is in a cold runner system, the hydraulic diameters of runners at different branches of a hot runner system may be more uniform than the progressively-smaller diameters of a cold runner system with increasing proximity to the mold cavity.
In describing a runner system, it is useful to consider the following terms: The term “main sprue” refers to the first runner leg that is adjacent to the machine nozzle and receives molten polymer (also referred to herein as molten polymeric material or thermoplastic material) from the molding machine. For a multi-cavity mold, the main sprue is divided into multiple “runner branches”, such that the number of “final runner branches” is equal to the total number of gate locations (usually one gate per mold cavity). The term “runner branch” refers to each of the flow channels in a runner network. The term “final runner branch” refers to the runner branches that connect directly to the gate, which then connects to the mold cavity. The term “node” refers to a location in the runner network where a runner is divided into smaller runner branches. For example, when the main sprue is divided into four runner branches extending out to four individual mold gates, the intersection of the main sprue with the runner branches is referred to herein as a “node”.
For a conventional molding process, the size of each of the runner branches is related using the formula Dm=Db*N1/3, where N is the number of runner branches extending from a feeder branch [Dm]. N is equal to the number of times a feed runner [Dm] is divided into equal runner branches [Db]. Dm and Db are hydraulic diameters.
For example, for a runner system where the main sprue is divided into four branches to feed four final runners, N would equal 4. Thus, where Db is equal to 6 millimeters, Dm is equal to 6*41/3 power, or Dm is equal to about 9.524 millimeters.
In a second example, a runner system where the main sprue is divided into four equal branches, and each of the four equal branches is then divided into four equal final runner branches. The diameter of the main sprue would be determined by starting with the diameter of the final runner, then working back through the system to the main sprue. Thus, where the final runner diameter [Db] is equal to 6 millimeters, the feeder runner diameter [Dm] is equal to 6*41/3 power, or is equal to about 9.524 millimeters. The next feeder runner, which in this case would be the main sprue diameter, would then be calculated in the same manner starting with the diameter of about 9.524 millimeters. Thus, the diameter of the main sprue [Dm] is equal to 9.524*41/3, or 15.118 millimeters. An equivalent calculation is Dm=Db*[the total number of final runners]1/3. For example, the 16 cavity tool indicated in second example above, if calculated by this formula provides the same answer of 15.118 millimeters. Specifically 6 mm*161/3 equals 15.118 millimeters. This relationship holds true regardless of the numbers of nodes located between the main sprue and the final runners. Each interim runner branch step would be related by the formula Dm=Db*N1/3.
This results in a substantial volume of plastic being required to distribute the polymer to the injection mold cavities. In the case of a cold runner system, this large volume can extend the cycle times for some parts, increase clamp tonnage (because the larger the volume of the runner system, the higher the volume of polymer material between the machine nozzle and the mold cavities, and the more clamp tonnage that may likely be necessary to mold articles along with the volume of runner material), and this substantial volume of polymer is typically disposed of for each “shot” of polymer injected in to the cavity—since the cold runner is typically discarded as scrap or reground for re-use in subsequent injection molding cycles. In the case of a hot runner system, this volume of material is heated during each molding cycle, thus the higher the volume of the runner, the longer the polymer residence time, and the longer the polymer is exposed to heat that degrades the polymer. Furthermore, the more volume of material contained in the hot runner, the more material that must be purged from the system when changing the color of a polymer or changing from one polymer material to another polymer material. This leads to lost productivity during the material changeover process. For both hot and cold runners, it is desirable to reduce the total volume of material contained in the runner.